1. Field of the Invention
This invention relates in general to filter circuits and in particular to a novel charge transfer device consisting of individual CTD elements formed into four-terminal quadripole resonators which are designed as self-contained closed looped circuits which determine the frequency-dependent transmission behaviour of the filter circuit and wherein successive four-terminal resonators are interconnected by way of switching coupling circuits.
2. Description of the Prior Art
Filter circuits utilizing charge transfer devices are known as, for example, as described in German LP No. 2,453,669. Specific embodiments of such circuits are also described in German AS No. 2,534,319 and German AS No. 2,608,582. Also, see the article from Wireless World, February 1975, pages 61 through 65 entitled "Charge-Coupled Devices-Signal Processing" by D. J. MacLennan of the University of Edinburgh. In these publications, it is described that bucket brigade devices and charge coupled devices are designated as CTD lines. CCD circuits are installations which operate according to the principle of coupled charges. For these two types of circuits, BBD and CCD, the designation of "CTD-installation" (charge transfer device) has been adopted in the electronic technical language and such devices also have the characteristic that when used as a transmission device they are unidirectional. CTD devices consist of a greater integral number, for example, n individual CTD elements which can be constructed in the form of fully integratable arrangements. It is known that such CTD arrangements must be operated by way of clock pulse signals with a particular clock pulse frequency f.sub.T which clock pulse signals are supplied to the individual charge transfer capacitors of the BBD arrangements or to the transfer capacitances of the CCD lines. In practice, generally multiphase CTD arrangements are preferred over a single phase devices and the clock pulse signals are phase-displaced relative to each other so that adjacent transfer capacitances are operated with phase displaced clock pulses. Consequently, if a so-called p-phase clock pulse system (p-2, 3, 4 . . . ) is used then one CTD element consists of p adjacent transfer capacitances. For additional discussion of such devices refer to the book entitled "Charged Transfer Devices" Academic Press, Inc., New York, San Francisco, London, 1975.
Filter circuits constructed as described in German LP No. 2,453,669 or German AS No. 2,534,319 or according to German AS No. 2,608,582 utilize as the resonant structures self-contained or closed looped circuits which are designated in this application as four-terminal resonators. In German AS No. 2,534,319, input and output coupling circuits are additionally illustrated which are designed in the form of lattice filter section circuits wherein the properties of the CTD arrangements are considered when constructing the circuit. In the input and output coupling circuits illustrated in German AS No. 2,608,582 the filter is formed with .pi. sections.
The ratio of the transfer capacitance of the CTD devices utilized in the supply lines relative to the transfer capacitance of the CTD arrangements in the closed looped circuits utilized in the four-terminal resonator assists in determining the filter characteristics of the device.
Although the known filter circuits require only a comparatively small space since they are formed in integrated techniques, it is desirable to form such circuits with the smallest possible number of CTD elements.